The process of growth is dependent on the maintenance of a positive balance for a variety of substances, including phosphate. Clearance studies indicate that the kidney contributes to this positive balance by maintaining TRP at a level higher than that observed in adult subjects having similar plasma phosphate concentrations. Analysis of the relationship between plasma phosphate level on the one hand, and PTH in growth hormone on the other, suggests that the latter plays an important role in maintaining tubular proximal reabsorption of phosphate at this relatively high level, which is appropriate to the needs of the growing organism. The studies described in this application are designed to determine the influence of various modulating factors and to determine the relative contribution of the proximal and distal tubule to the absorption of phosphate in the developing animal. The isolated perfused kidney will be used to assess the role of phosphate concentration, PTH, growth hormone, sodium and glucose on the reabsorption of phosphate in guinea pigs of various ages, while micropuncuture studies will permit to quantitate the fraction of the filtered phosphate absorbed in various segments of the renal tubule of these animals. The results of these studies will advance our understanding of one of the basic phenomenon observed in developmental physiology: the achievement of a positvive balance for essential substances in the presence of significant limitations in the intrinsic capacity for their transport.